The present invention relates to a method for monitoring the nitrogen oxide storage capacity of a nitrogen oxide storage catalyst used as a starting catalyst in an exhaust-gas purification system of a motor vehicle with a lean-burn engine, the system including the starting catalyst and a main catalyst, which is likewise designed as a nitrogen oxide storage catalyst.
To reduce the fuel consumption of petrol engines, what are known as lean-burn engines which are operated with lean air/fuel mixes in the part-load range have been developed. A lean air/fuel mix contains an oxygen concentration which is higher than necessary for complete combustion of the fuel. In this case, the corresponding exhaust gas contains the oxidizing components oxygen (O2), nitrogen oxide (NOx) in excess compared to the reducing exhaust-gas components carbon monoxide (CO), hydrogen (H2) and hydrocarbons (HC). Lean exhaust gas usually contains 3 to 15% by volume of oxygen. However, when operating at load and full load, a stoichiometric or even substoichiometric, i.e. rich, air/fuel preparation is realized even in lean-burn spark-ignition engines.
By contrast, diesel engines generally operate under conditions with highly superstoichiometric air/fuel mixes. Only in recent years have diesel engines which can also be operated with rich air/fuel mixes for a short period of time been developed. Diesel engines, in particular those with the option of rich operating phases, are also encompassed by the term lean-burn engines in the context of the present invention.
On account of the high oxygen content of the exhaust gas from lean-burn engines, the nitrogen oxides contained therein cannot be continuously reduced to form nitrogen, with simultaneous oxidation of hydrocarbons and carbon monoxide, with the aid of three-way catalysts as used in spark-ignition engines operated under stoichiometric conditions. Therefore, what are known as nitrogen oxides storage catalysts, which store the nitrogen oxides contained in the lean exhaust gas in the form of nitrates, have been developed for the purpose of removing the nitrogen oxides from the exhaust gas from these engines.
The operation of nitrogen oxide storage catalysts is described extensively in SAE document SAE 950809. According to this, nitrogen oxide storage catalysts consist of a catalyst material, which has generally been applied in the form of a coating to an inert honeycomb carrier made from ceramic or metal, referred to as a carrier. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material in turn consists of the actual nitrogen oxide storage component, which has been deposited in highly disperse form on a support material. The storage components used are predominantly the basic oxides of the alkali metals, the alkaline-earth metals and the rare earths, but in particular barium oxide, which react with nitrogen dioxide to form the corresponding nitrates.
The catalytically active components used are usually the precious metals from the platinum group, which are generally deposited together with the storage component on the support material. The support material used is predominantly active alumina with a high surface area. However, the catalytically active components may also be applied to a separate support material, such as for example active alumina.
The role of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas into carbon dioxide and water. Moreover, they are intended to oxidize the nitrogen monoxide content of the exhaust gas to form nitrogen dioxide, so that it can react with the basic storage material to form nitrates (storage phase or lean-burn mode), since the nitrogen oxides in the exhaust gas from lean-burn engines, depending on the operating conditions of the engine, are made up of 65 to 95 Vol.-% of nitrogen monoxide, which cannot react with the storage components.
In addition to the abovementioned components, the nitrogen oxide storage catalyst may also contain components that store oxygen. In this case, in addition to the storage of nitrogen oxides, it can also perform functions of a conventional three-way catalyst. For the most part, the oxygen-storing component used is cerium oxide. In addition to its nitrogen oxide storage function, the nitrogen oxide storage catalyst also has an oxygen storage function, meaning that it therefore has two functions.
As the accumulation of nitrogen oxides in the storage material increases, the storage capacity of the material decreases and there is more and more slippage of nitrogen oxides through the storage catalyst. Therefore, the storage catalyst has to be regenerated from time to time. For this purpose, the engine is briefly operated with air/fuel mixes with a stoichiometric or rich composition (during what is known as the regeneration phase or rich-burn mode) Under the reducing conditions in the rich exhaust gas, the nitrates which have formed are decomposed to form nitrogen oxides NOx, which are reduced, using carbon monoxide, hydrogen and hydrocarbons as reducing agents, to form nitrogen together with water and carbon dioxide.
When the nitrogen oxide storage catalyst is operating, the storage phase and regeneration phase alternate at regular intervals. The storage phase usually lasts for between 60 and 120 seconds, whereas the regeneration phase is terminated after less than 20 seconds. It is customary for a nitrogen oxide sensor to be arranged downstream of the storage catalyst in order to determine the optimum instant for switching the engine from the storage phase to the regeneration phase. If the nitrogen oxide concentration in the exhaust gas measured by this sensor rises above a preset threshold value, regeneration of the catalyst is initiated. The nitrogen oxide concentration in the exhaust gas is therefore used as a criterion for initiating regeneration.
Modern nitrogen oxide storage catalysts have a working range of between approximately 150 and 500° C., i.e. below this temperature the storage catalyst can no longer store the nitrogen oxides contained in the exhaust gas in the form of nitrates, since its catalytically active components are not yet able to oxidize the nitrogen oxides to form nitrogen dioxide. Above 500° C., the nitrogen oxides stored as nitrates are thermally decomposed and released to the exhaust gas as nitrogen oxides.
One significant problem in modern exhaust-gas purification methods is that of checking that the catalysts used are functioning correctly, in order to allow catalysts which are no longer functional to be replaced in good time. This also applies to nitrogen oxide storage catalysts, the nitrogen oxide storage capacity of which may be damaged on the one hand by the sulphur which is present in the fuel and on the other hand by thermal overloads. Whereas poisoning by sulphur can generally be reversed by regeneration at elevated temperatures, thermal damage is irreversible.
In the case of storage catalysts with two functions, in principle both storage functions may be damaged by poisoning and thermal influences. In this case, the damage to one function does not necessarily mean damage to the other function. Since nitrogen oxides and oxygen are both oxidizing components, it is not possible to clearly distinguish their effects from one another, and consequently misdiagnoses may occur when testing the catalyst. DE 198 16 175 A1 describes one possible way of separately assessing the two storage functions with the aid of an oxygen sensor arranged downstream of the storage catalyst.